The field of the disclosure is related to superconducting circuits. More particularly, the disclosure relates to systems and methods for controlling superconducting qubits using single flux quantum (“SFQ”) circuits.
In the field of quantum computation, the performance of quantum bits (“qubits”) has advanced rapidly in recent years, with several preliminary multi-qubit initiatives aiming toward surface code architectures. In contrast to classical computational methods that rely on binary data stored in the form of definite on/off states, or classical bits, qubits take advantage of the quantum mechanical nature of quantum systems to store and manipulate data. Specifically, quantum systems can be described by multiple quantized energy levels or states, and can be represented probabilistically using a superposition of those states.
Among several implementations currently being pursued, superconductor-based qubits present good candidates for quantum computation. This is because superconducting materials have inherently low dissipation that, in principle, can produce coherence times necessary for performing useful calculations. For instance, qubits based on Josephson tunnel junctions, which include two superconducting electrodes separated by a thin insulator, are advantageous due to their strongly nonlinear behavior. Specifically, Josephson-based devices allow for breaking the degeneracy between different transition frequencies, and thereby restrict system dynamics to specific quantum states. In addition, complex superconducting circuits can be micro-fabricated using conventional integrated-circuit processing techniques. This allows scaling to architectures that include a large number of qubits.
A fault-tolerant scalable quantum computer can provide a computational power far exceeding that of a classical computer, and superconducting qubits are a promising way to build such a machine. However, large-scale quantum information processing based on surface codes imposes strict challenges on qubit operation and control. For instance, by some estimates, a general-purpose fault-tolerant quantum computer will likely include millions of physical qubits. Using current implementations, controlling such large-scale quantum computer through qubit manipulation, error detection, and readout, would involve a massive hardware overhead.
Conventionally, qubits are controlled using pulses generated by single-sideband modulation of a microwave carrier tone. Accurate control of both the in-phase and quadrature pulse amplitudes allows arbitrary rotations on the Bloch sphere. However, utilizing microwave pulses introduce the possibility of crosstalk between neighboring qubit channels of a qubit array. To minimize crosstalk, different qubits in the array are often biased at different operating frequencies. This approach also makes it possible to address a large-scale multi-qubit array with a relatively small number of carrier tones, which results in significant hardware savings.
In some approaches, control waveforms are recycled and used across a qubit array. However, it is not clear that recycling waveforms allows high-fidelity control. This is because such waveforms represent the convolution of the applied waveforms and transfer functions of the wiring in the cryostat system. However, transfer functions are generally not well controlled, and can vary substantially across the array. Moreover, having separate high-bandwidth control lines for each qubit channel entails a massive heat load on the milli-Kelvin stage of the cryostat system. Furthermore, the significant latency associated with the round trip signal travel from the quantum array to the room-temperature classical coprocessor will limit the performance of any scheme used for high-fidelity projective measurement and feedback to stabilize the qubits in the array.
Given the above, there exists a need for systems and methods yielding scalable quantum computation that includes the ability to perform rapid high-fidelity control and measurement of both single qubits and multi-qubit parity, while controlling the resources utilized.